1. Field of the Invention
The present invention relates to an optical information recording medium, and more particularly to an information recording medium including a light absorbing recording layer including a dye.
2. Discussion of the Related Art
Recently, recordable compact discs such as CD-Rs and CD-RWs have been developed for commercial operation, and are widely used as well as read-only compact discs (CDs).
Information can be additionally recorded in CD-Rs and CD-RWs whereas information cannot be recorded in CDs. The information signals recorded in CD-Rs and CD-RWs fulfill the specifications of conventional CDs and therefore the recorded information can be reproduced by marketed CD players.
Published Japanese Patent Application No. 2-42652 discloses a CD-R which is prepared by spin-coating a dye on a substrate to form a light absorbing recording layer thereon and then forming a metal reflection layer on the recording layer. Dyes are typically used for such a light absorbing recording layer.
The reason why dyes are used for the light absorbing. recording layer is as follows. CD-Rs have to have a high reflectance not less than 65%, which is one of the specifications of CDs. In order that such a CD-R as having the above-mentioned constitution has such a high reflectance, the light absorbing recording layer thereof needs to have a specific complex index of refraction at the wavelength of the recording/reproducing light. Dyes have light absorption characteristics suitable for such a light absorbing layer.
The light absorption spectrum of the optical information recording medium using a dye is illustrated in FIG. 1. The complex index of refraction utilizes the characteristics of the edge portion of the absorption band of the spectrum. However, dyes have a drawback in that the complex index of refraction has a large wavelength dependency.
Recently, in order to perform high density recording, a need exists for an optical information recording medium (DVD-Rs and DVD+Rs) in which information can be recorded or reproduced using laser light having a wavelength of about 650 nm which is shorter than the wavelength (780 nm) of laser light typically used for recording/reproducing information in conventional CDs. The light absorption of the dyes mentioned above for use in the conventional CD-Rs, such as pentamethine cyanine dyes and phthalocyanine dyes, have so large wavelength dependency that the dyes cannot have good recording/reproducing characteristics (i.e., information cannot be recorded and reproduced) when laser light having a wavelength of 650 nm is used for information recording and reproducing. This is because the dyes have a large absorption coefficient k, i.e., a small reflectance, at the wavelength of 650 nm.
In order to provide a dye having suitable light absorption property, various dyes such as trimethine cyanine dyes, azo dyes and tetraazaporphyradine dyes have been proposed in, for example, Published Japanese Patent Applications Nos. 9-169166, 9-66671 and 11-48612.
These dyes have good optical characteristics at the wavelength of 650 nm, but do not have good optical characteristics against light having a wavelength in a range of from 645 to 670 nm, which light is typically used for DVD+R drives. Namely, when the wavelength of the laser light emitted by a DVD drive changes due to changes of the driving conditions such as environmental conditions, a problem in that information cannot be recorded in the recording medium tends to occur. In particular, since the wavelength of laser light emitted by a light source tends to shift in the long wavelength direction under high temperature conditions, the absorption coefficient k decreases and thereby the recording sensitivity of the recording medium deteriorates.
In order to decrease the wavelength dependency of the recording sensitivity, Published Japanese Patent Application No. 2001-26180 discloses an optical information recording medium in which a mixture of a dye having a maximum absorption at a wavelength of from 500 to 620 nm, another dye having a maximum absorption at a relatively long wavelength and yet another dye having a maximum absorption at a relatively short wavelength is used in the recording layer thereof. However, the application does not pay attention to the wavelength (i.e., 645 to 670 nm) of the light emitted by DVD drives, and therefore the wavelength dependency of the recording sensitivity cannot be improved.
DVDs have a recording format different from that of CDs. In addition, the track pitch length and minimum pit length of DVDs are shorter than those of CDs. By narrowing the light spot of reproduction light, information can be recorded in DVDs at a density higher than that for CDs by about 6 to 8 times. This is because DVDs have a constitution different from that of CDs.
Specifically, CDs have the following constitution:                Thickness of substrate: about 1.2 mm        Wavelength of reproduction light: 780 nm        NA: 0.45 to 0.5        Track pitch: about 1.6 μm        Minimum pit length: about 0.8 μm        
In contrast, DVDs have the following constitution:                Thickness of substrate: about 0.6 mm        Wavelength of reproduction light: 650 nm        NA: 0.6 to 0.65        Track pitch: about 0.74 μm        Minimum pit length: about 0.4 μm        
Such high density recording, i.e., to record a pit much smaller than that for CDs, cannot be achieved by only adjusting the optical characteristics, and it is needed to adjust the heat decomposition properties of the dyes used. In addition, it is needed to solve a problem in that the jitter of recorded signals is large.
Because of these reasons, a need exists for an optical information recording medium which can be used as a DVD-R or a DVD+R and which has good recording characteristics with a small wavelength dependency.